In recent years, from the standpoint of environmental issues and energy issues, vehicles utilizing electric motors as driving sources are attracting attention.
A driving source used in a vehicle is required to generate a large driving torque during a low speed travel, e.g., when the vehicle pulls out or goes up a slope, etc. During a steady travel, on the other hand, the driving torque may be small but rapid rotation is required. In other words, an electric motor which is capable of changing its output characteristics is required.
For example, the specification of Japanese Patent No. 2749560 discloses a technique of changing the characteristics of a motor by adjusting the gap between the teeth of a stator and the magnet of a rotor.
The motor of this technique comprises, as shown in FIG. 1, a drum-like rotor 125, a stator 130, and a fastening/adjustment member 160 for adjusting the gap between a bush 146 and the drum-like rotor 125. If a head 162 of the fastening/adjustment member 160 is manipulated so as to loosen the fastening/adjustment member 160, the elastic member 161 causes the bush 146 to be spaced apart from the stator 130. As a result, a gap G between a magnet 141 of the rotor 125 and the stator 130 can be increased. On the other hand, by tightening the fastening/adjustment member 160, the gap G between the magnet 141 and the stator 130 can be reduced. The disclosure states that, through adjustments by changing the gap G, it becomes possible to inexpensively produce a motor which is suitable for many specifications.
Japanese Laid-Open Patent Publication No. 3-215154 discloses a technique, in a motor for rotation-driving the reels of a tape recorder, of adjusting the gap between the rotor and the stator. This technique causes an electric current flowing in an electromagnet to change in proportion with an electric current flowing in the motor, thus adjusting the gap with the magnetic force of the electromagnet and a spring which is attached to the axis of the motor.
On the other hand, Japanese Laid-Open Patent Publication No. 9-37598 discloses a technique of changing the power generation characteristics of a motor which is used as an electric generator. According to this technique, the amount of generated power can be adjusted by adjusting the overlap between the magnet of the rotor and the coil of the stator.
However, a motor of any such conventional technique may have difficulties in changing its output characteristics during the motor rotation, or may be unsuitable for use as a driving source of a vehicle.
Moreover, as compared to an internal combustion engine, an electric motor permits a large freedom with respect to its outer shape, and motors of various shapes can be realized. Therefore, in the case where an electric motor is used as a driving source of a vehicle, it is possible to make the outer shape of the driving source small, or design an outer shape which is in accordance with the space that is allocated for the driving source, for example by placing the driving source within the wheel of a tired wheel, as compared to the case of using an internal combustion engine. However, in the case where there are limits to the shape and size of the mechanism for causing changes in the output characteristics of the electric motor, such advantages of an electric motor may be undermined.
Furthermore, in the case where an electric motor is used as a driving source of a vehicle, and a battery is used to cause the electric motor to rotate, the weight of the battery is large relative to the battery capacity, so that it is preferable that the electric motor has a high energy efficiency for making the travel distance of the vehicle as long as possible. Therefore, the mechanism for causing changes in the output characteristics is required to not introduce a large reduction in the energy efficiency of the electric motor.
Particularly in the case where an electric motor is used as a driving source of a two-wheeled vehicle, it is desirable that as many such problems are solved as possible.